Two year breaks profitably reduce agronomic
constraints in the Northern Victorian Mallee
Michael Moodie1, Nigel Wilhelm2, Roger Lawes3, Peter Telfer2, Todd McDonald1
1MSF Mildura; 2SARDI Waite campus Adelaide, 3CSIRO Perth
Why was the trial/project undertaken?
The Low Rainfall Crop Sequencing project commenced in 2011. At that point
in time, crop sequences in the low rainfall region were dominated by
intensive continuous cereal cropping and break crops occupied less than 5 percent of the landscape.
These intensive cereal cropping sequences were declining in productivity with the emergence of
agronomic constraints such as grass weeds, low soil nitrogen and crop diseases. The aim of this project
was to test if including one and two-year break phases in low rainfall crop sequences could remove
the agronomic constraints and increase the production of subsequent cereal crops. We also explored
whether break crops could improve the profitability of the long term crop sequence when compared
to maintaining continuous cereal.
How was the trial/project done?
The Mildura trial was located in the Millewa region of the Victorian Mallee. In 2011, nine different
break options were established along with continuous wheat. In 2012, a second break phase was
implemented (2-year break) or the rotation was returned to wheat (1-year break). In 2013, all
rotations were returned to either conventional wheat (var. Shield) or Clearfield wheat (var. Grenade).
In 2014, all plots were again sown to wheat (var. Grenade).
Throughout the trial, agronomic management was varied for each individual rotation to help maximise
the profitability of that rotation and to respond to particular issues in each option. For example
nitrogen inputs, varieties, sowing dates or herbicide applications were varied depending on the level
and type of agronomic constraints in each rotation.
Key Messages
• Including 1 and 2-year break phases in the low rainfall Mallee can significantly increase the
productivity of subsequent wheat crops.
• Brome grass population in a long term cereal paddock near Mildura was the most significant
driver of the break benefits in a crop sequencing trial.
• Including a two-year break phase in the rotation was up to $90/ha/year more profitable than
maintaining continuous wheat over the four year period of the trial.
Background
The Low Rainfall Crop Sequencing project commenced in 2011 with field trials at 5 sites (Minnipa (SA),
Appila (SA), Mildura (Vic), Chinkapook (Vic) and Condobolin (NSW)) across the low rainfall zone of
south eastern Australia. At that point in time, crop sequences in the low rainfall region were
dominated by intensive cereal cropping and break crops occupied less than 5 percent of the landscape.
Moreover these intensive cereal cropping sequences were declining in productivity due to agronomic
constraints such as grass weeds, declining soil nitrogen fertility and crop diseases.
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The aim of this project was to test if including one or two-year break phases in low rainfall crop
sequences could successfully address agronomic constraints to increase the productivity of
subsequent cereal crops and improve the profitability of the long term crop sequence when compared
to maintaining continuous cereal. In this article we report on the yields and break crop effects
observed in 20 different crop sequences and the profitability of the rotations over four seasons of the
project (2011-2014) at the Mildura trial site.
About the trial
The Mildura trial was located in the Millewa region of the Victorian Mallee. In 2011, nine different
break options were established along with continuous wheat. In 2012, a second break phase was
implemented (2-year break) or the rotation was returned to wheat (1-year break). In 2013, all
rotations were returned to either conventional wheat (var. Shield) or Clearfield wheat (var. Grenade).
In 2014, all plots were again sown to wheat (var. Grenade).
Long term average rainfall at Mildura is 290 mm with approximately two thirds falling in the growing
season (April to October) (Table 1). In 2011, record rainfall (444 mm) fell between January and March,
filling the soil moisture profile prior to sowing. However only a further 108 mm of rainfall was received
in the growing season. Very low growing season rainfall (92 mm) was also received in 2012 which also
had a very late ‘season break’ with significant opening rains not received until July. Average growing
season rainfall was received in the 2013 and 2014.
Throughout the trial, agronomic management was varied for each individual rotation to help maximise
the profitability of that rotation and to correct the agronomic constraints that emerged for that
rotation. For example nitrogen inputs, varieties, sowing dates or herbicide applications were varied
depending on the level and type of agronomic constraints in each rotation.
Gross Margins were calculated for each treatment in each season using the ‘Farm Gross Margin and
Enterprise Planning Guide’ (Rural Solutions 2011, 2012, 2013, 2014 and 2015). Costs were calculated
using the actual inputs used in the trial and the values provided in the corresponding gross margin
guide. The commodity price stated for January in the year after yield was measured was used to
calculate the income (e.g. 2011 yield x January 2012 price). Treatment grain yields were used for
calculating income and 85% of dry matter yield was used for calculating hay yield. For pastures,
income was calculated using the dry sheep equivalent (DSE) cereal zone gross margin for a prime lamb
enterprise and a nominal stocking rate of 2 DSE per winter grazed hectare, irrespective of pasture
production.
Table 2. Monthly rainfall (mm) at Mildura Airport for the duration of the trial. Mean is the long term
average rainfall.
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Annual
Mean
22
23
21
18
25
22
26
26
27
29
26
26
290
2011
129
193
122
13
14
11
15
21
7
28
43
62
657
2012
13
37
64
4
3
8
41
17
13
6
5
4
215
2013
1
15
11
6
29
36
15
10
19
14
2
58
216
2014
1
67
29
58
23
6
13
18
19
1
13
10
257
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Results
Treatment Yields
Grain yields achieved from each treatment over the four years of the trial are presented in Table 2.
Crop yields were slightly above district expectations in 2011 with field peas the standout crop at the
site. The poorest crop yields were achieved in 2012 due to very low growing season rainfall and a late
break. In the 2013 and 2014 seasons when the whole site was sown to wheat, the yields of the
continuous wheat treatment were in line with district expectations, however large break effects were
observed in many of the treatments where break phases were previously implemented.
Dry matter (DM) yield was also measured for non-grain crops. In 2011, DM production was high across
all crops with Oaten Hay yielding 4.8 t/ha, vetch producing 2.7 t/ha and medic producing 3.2 t/ha. A
late break and low in-crop rainfall in 2012 constrained the dry matter production of vetch (0.99 t/ha).
However, volunteer medic pasture which was able to germinate after 64 mm of rainfall in march (Table
2) accumulated very high dry matter yields (7.1 t/ha) despite the poor growing season rainfall.
Table 2. Grain yields for grain crops over four years (2011-2014) at the Mallee Crop Sequencing Trial
2011 Crop
Yield
t/ha
2012 Crop
Yield
t/ha
Canola (TT)
0.93
Chickpea
Canola (TT)
0.62
Canola (TT)
Chickpea
Chemical Fallow
Chemical Fallow
2013 Crop
Yield
t/ha
b
0.30
Wheat
2.57
Ix
1.41
Field peas
1.17
Wheat
2.44
Ix
1.43
0.66
Vetch (Brown
Manure)
NA
Wheat
2.54
Ix
1.54
0.83
Canola (TT)
0.39
Wheat
2.31
Ix
1.61
2.27
Ix
1.52
2.42
Ix
1.20
1.60
NA
NA
b
Canola (Clearfield)
Chemical Fallow
b
0.27
Wheat
NA
Wheat
2014 Crop
Wheat CL
Wheat CL
Wheat CL
Wheat CL
Wheat CL
Wheat CL
Yield
t/ha
Chemical Fallow
NA
Field peas
1.24
Wheat
2.50
Ix
Medic (high seed
rate)
NA
Medic (volunteer)
NA
Wheat
1.96
Ix
1.43
Medic (low seed rate)
NA
Medic (volunteer)
NA
Wheat
2.19
Ix
1.54
1.33
Wheat CL
Wheat CL
Wheat CL
Field pea
1.35
Canola (TT)
0.39
Wheat
2.32
Ix
Field pea
1.93
Vetch (Brown
Manure)
NA
Wheat
2.67
Ix
1.57
NA
Canola (TT)
0.25
Wheat
2.58
Ix
1.84
NA
Field pea
0.77
Wheat
2.60
Ix
1.83
2.2
Wheat
0.76
Wheat CL
1.04
Ix
1.78
1.44
Ix
1.17
1.00
Ix
1.69
1.22
Ix
1.69
1.12
Ix
1.69
1.30
Ix
1.65
1.42
Ix
1.31
Vetch (Brown
Manure)
Vetch (Brown
Manure)
Barley
Canola (Clearfield)
Canola/Field pea mix
Oaten hay
Field peas
Chemical Fallow
0.59
Wheat CL
1.17
Wheat
NA
Wheat
2.05
Wheat
NA
Wheat
1.47
LSD (Grain Yield Only)
0.44
b
Ix
Wheat
Ix
Wheat CL
Wheat CL is Clearfield wheat
Intervix applied to the Clearfield wheat
Ix
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1.05
Ix
Wheat CL
0.87
Ix
0.82
Ix
1.26
Ix
1.26
Ix
0.93
0.25
Wheat CL
Wheat CL
Wheat CL
Wheat CL
Wheat CL
0.20
Wheat CL
Wheat CL
Wheat CL
Wheat CL
Wheat CL
Wheat CL
Wheat CL
Wheat CL
Wheat CL
Wheat CL
Wheat CL
0.20
Break effects
Including break phases in the rotation increased the productivity of subsequent cereal crops relative
to maintaining continuous wheat. A one-year break phase of field pea or fallow in 2011 led to an
increase of 0.3 t/ha and one year of canola resulted in a 0.1 t/ha yield benefit in the 2012 wheat crop.
In 2013, the benefit of having a two-year beak prior to 2013 was 0.5-1.25 t/ha. However the benefit
of the 1-year break crop options from 2011 only lasted a single season. Two-year break crop benefits
were also observed in 2014 with selected rotations having up to a 0.4 t/ha greater yield than
continuous wheat.
The importance of brome grass, soil nitrogen, rhizoctonia and soil water to the yield difference
between break options and continuous wheat was analysed for wheat yields in 2013 and 2014 (Figure
1).
2013 was the first year following a two-year break and second year following a one-year break.
Averaged across all positive break effects, 39 percent of these yield increases were due to less brome
grass, 38 percent to more nitrogen, 19 percent to less rhizoctonia and four percent to more water.
Where negative break effects were observed, 77 percent of the difference was due to increased
brome grass, 15 percent to increased rhizoctonia, six percent to less water and two percent to less
nitrogen.
Brome grass was the dominant driver of positive break effects in 2014 accounting for an average of
80 percent of the differences in wheat yield. Higher soil nitrogen levels accounted for a further 18
percent of the positive break effect. Where continuous wheat in 2014 out yielded wheat following a
break option, nitrogen accounted for
58 percent and brome grass 37
percent
of
the
difference.
Rhizoctonia and soil water accounted
for very little of the yield differences
observed in the trial in 2014.
Figure 1. Contribution of Brome
grass (in-crop), mineral nitrogen
(pre-sowing mineral 0-60 cm),
rhizoctonia (pre-sowing inoculum)
and soil water (pre-sowing 0-120 cm)
to break effect (difference between
treatment and continuous cereal
yield in the Mildura trial in 2013 and
2014). A positive effect means that
the difference in the level of the
agronomic constraint increased the
yield of the break relative to
continuous wheat and a negative
effect means the level of the
agronomic constraint resulted in less
yield in the break option than in the
control.
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Long term profit
Gross margins were calculated for all treatments in each year and then accumulated for each
sequence for the duration of the trial (Figure 2). Over four seasons, 15 of the 19 rotations were more
profitable than maintaining continuous wheat. On average, the top five most profitable rotations
were $360/ha or $90/ha per year more profitable than continuous wheat. Furthermore, four of the
top five most profitable rotations included two years of a break. Key characteristics of the most
profitable rotations that included a two-year break were the inclusion of at least one profitable break
crop in the sequence and the alleviation of agronomic constraints resulting in increased profitability
in the wheat crops following the break phase.
Of the rotations that were less profitable than wheat, three of the four included at least one fallow
phase. The other rotation was a two-year pasture, which had a high brome grass population because
the only weed control tactic was to spray top. However, the absolute difference in gross margin
between these treatments and the continuous wheat treatment was small (on average less than
$40/ha or $10/ha/year).
Figure 2. Seasonal
gross
margins
(2011-2014)
for
each treatment in
the low rainfall crop
sequencing trial site
at Mildura.
Implications for commercial practice
This project has demonstrated that diversified cropping sequences have an important role to play in
low rainfall Mallee farming systems. Including 1- and 2-year break phases were shown to significantly
increase the productivity of subsequent wheat crops. In this trial, the greatest break crop benefits
immediately following the break phase were due to reduced weed number and improved nitrogen
status. Reduced rhizoctonia inoculum and additional stored soil water were important in select
rotations, although the benefits were much smaller than for addressing grass weed and nitrogen
constraints. In the second wheat crop, grass weed numbers were the major driver of the yield
differences between treatments.
This project showed that including a two-year break phase in the rotation was often more profitable
than maintaining continuous wheat. These profitability benefits were significant, with the best
treatments generating $90/ha/year more profit than the continuous wheat treatment. A key driver
of increased profit was to have at least one profitable break crop in the rotation. Therefore, future
research should focus on finding low cost, reliable and profitable break phase options for farmers in
low rainfall regions such as the Mallee.
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Links and references
Rural Solutions SA (2011, 2012, 2013, 2014, 2015). Farm Gross Margin and Enterprise Planning Guide
(Years 2011, 2012, 2013, 2014, 2015).
http://solutions.pir.sa.gov.au/news/news/newspaper_items/2014/farm_gross_margins_and_enterp
rise_planning_guide_2015
Acknowledgements
This trial is a collaboration between MSF and SARDI with funding from the GRDC.
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